: Malaria is recognized as the most widespread parasitic infection in humans. Primaquine has been a major antimalarial drug for over 40 years due to its unique effectiveness against exoerythrocytic forms of the parasite. Its therapeutic value has grown in recent years with the development of resistance to alternate antimalarial drugs such as chloroquine and because of its utility in the treatment of Pneumocystis carinii pneumonia in AIDS patients. However, primaquine therapy has been severely limited because of its capacity to induce methemoglobinemia and hemolytic anemia, particularly in patients with glucose-6-phosphate dehydrogenase deficiency. It has long been known that the hemotoxicity of primaquine is due to the action of metabolites and not the parent compound. However, the toxic specie(s) have not been identified and little is known about the mechanism underlying red cell injury. In collaborative studies with individuals at Walter Reed, we have examined the hemotoxicity of known and putative phenolic metabolites of primaquine and have observed potent and direct-acting hemotoxicity. In other studies, we have synthesized 6-methoxy-8-hydroxylaminoquinoline (MAQ-NOH) and found that this metabolite was also a direct-acting hemotoxicant. We now propose to investigate the mechanism of red cell damage induced by these metabolites and examine their metabolic formation in vitro. The hypotheses under test in this proposal are: 1) that two pathways of oxidative damage, initiated by lipid peroxidation and protein thiol oxidation, occur in the red cell; and 2) that quinone/quinoneimine metabolites act via lipid peroxidation, whereas the N-hydroxy metabolite acts via protein thiol oxidation. Three aims are presented: 1) to characterize the hemolytic response and pattern of oxidative injury induced within red cells by each type of primaquine metabolite; 2) to elucidate the oxidative metabolism of primaquine in rat and human liver microsomes and hepatocytes, and identify GYP isoforms responsible for primaquine metabolism; and 3) to identify intracellular and external cell surface alterations that correlate with phagocytosis of primaquine metabolite-damaged red cells by cultured splenic macrophages.